The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile, or an image forming apparatus which has a compound function, especially relates to a color image forming apparatus using developer including toner and carrier and equipped with a plurality of developing devices and to a process cartridge with quipped thereto.
Although colorization is progressing also in an image forming apparatus in recent years, when color image forming methods having high practicability are classified roughly, there may be four types, such as a transfer drum type, an intermediate transfer type, a KNC type (plural development one-time transfer type) and a tandem type, as a name often used.
Since the names of these types are named from a certain viewpoint, for example, there may be a method with both of the tandem type and the intermediate transfer type and there may be a method which directly transfers to a transfer material. Among these types, in the tandem type, that is, in a color image forming apparatus which forms each color image with a plurality of image forming units provided to each color and transfers each color one by one, kinds of available transfer material are too many, the quality of full color is very high and the full color can be obtained with high-speed. Especially, the special feature that the full color image can be obtained with high-speed is an advantage which is not seen in other types.
Moreover, although developer includes one component type developer and two component type developer, since the one component type developer does not need carrier, there is a merit that the cost of developer or a developing device can be made low. On the other hand, the two component type developer has a stable charging property of a toner, can obtain a high quality image, and is suitable also for a high-speed machine.
Furthermore, a smaller color image forming apparatus is requested due to the popularization of the color image forming apparatus, and in the color image forming apparatus using a plurality of developing devices, such as a tandem type, it is necessary to miniaturize each developing device more.
On the other hand, if the status of use of such a high-speed full color machine is taken into consideration, the colorization rate for a printing manuscript is not necessarily high and the machine is usually used only for a black printing manuscript in many cases. Therefore, there is a demand to increase the developer storage capacity of a developing device which stores a black developer.
In order to increase the black developer storage capacity, there is disclosed a color image forming apparatus having a structure in which a developing device which stores a black developer is made larger than developing devices which store developer of other colors (for example, refer to Japanese Patent Unexamined Publication Nos. 2001-183886 and 2001-75327).
The schematic diagram showing one example of a conventional color image forming apparatus shown in FIG. 14 is a schematic diagram of the color image forming apparatus disclosed by Japanese Patent Unexamined Publication No. 2001-183886, and the schematic diagram showing one example of a conventional color image forming apparatus shown in FIG. 15 is a schematic diagram of the color image forming apparatus disclosed by Japanese Patent Unexamined Publication No. 2001-75327.
In the color image forming apparatus disclosed by Japanese Patent Unexamined Publication No. 2001-183886 and shown in FIG. 14, our sorts of image forming stations are arranged side by side on a transfer belt arranged horizontally, and each image forming station is constituted with a photoreceptor drum, a charging device, a light exposing device, a developing device, and a cleaning means. The developing device has a structure in which a developing sleeve which includes a magnet roller therein, a RS roller (recovering and supplying member) which supplies a two component type developer to the developing sleeve and recoveries the developer and a toner feed member which agitates the developer are arranged side by side in a transverse direction. The developing device which stores a black developer is structured such that its size in the transverse direction is extended to longer than that of developing device of other colors and a toner supplying member is added in the extended space.
Namely, in the structure disclosed by Japanese Patent Unexamined Publication No. 2001-183886, four image forming stations are horizontally arranged side by side, and the developing device which stores the black developer and is arranged at the right end in FIG. 14 is constituted such that a new toner feed member is added in the transverse direction with the increase in a developer storing amount. Therefore, the developing device which stores a black developer becomes still more longer in the transverse direction, the whole image forming apparatus becomes longer and extremely large in the transverse direction, and it becomes difficult to attain the miniaturization of the whole image forming apparatus.
On the other hand, the color image forming apparatus disclosed by Japanese Patent Unexamined Publication No. 2001-75327 and shown in FIG. 15, [with three rollers] image forming stations are arranged side by side on an intermediate transfer belt which is arranged in three directions of vertical, horizontal and diagonal directions with three rollers. That is, three color image forming units (hereafter, merely refer as color unit) are arranged in the vertical direction, and a black image forming unit (hereafter, merely refer as a black unit) is arranged in the horizontal direction. By having arranged the black module independently, it has the feature that it makes easy to separate the color module from the intermediate transfer belt at the time of forming a black image and it can make the size of the developing device of a black unit larger while attaining the miniaturization of the image forming apparatus.
However, in the structure disclosed by Japanese Patent Unexamined Publication No. 2001-75327, the arranging position and the size of the developing device of the black unit are structured different from those of the color unit. Therefore, the magnetic pole arrangement of a developing roller and the structure of members in association with conveyance to supply developer to a developing roller or to recover developer have to be changed. It is Because the positional relationship of a developing pole in fixed magnetic poles of the developing roller positioned opposite to a photoreceptor drum is different between the developing device of the black unit and the developing device of the color unit, so the arrangements of an agitating roller to agitate developer and a feed roller to supply developer to the developing roller may change in connection with it.
Moreover, even if the developing device of a black unit and the developing device of a color unit are arranged in one line in a longitudinal direction, as far as the size of the developing device of the black unit is changed, the structure of members in association with conveyance of developer must be changed. Because, in order to increase the developer storing amount of the developing device of the black unit, it is necessary to enlarge the size of a developing device casing member. Further, in order to compensate shortage in abilities to agitate, convey and feed developer in connection with it, it is necessary to enlarge the size of an agitating roller or a feed roller. Even if the positional relationship between a photoreceptor drum and a developing roller is identically set among the developing devices of the black unit and the color unit, if the size of these rollers is enlarged, the conveyance passage or developer will change and in turn, the flow of conveyance and feed will change. Therefore, it is forced to change the arrangement of the fixed magnetic pole of the developing roller.
Furthermore, it may be predicted that a difference may arise in the changing way of the density level of an output image due to the difference in feeding and recovering actions for developer among the developing devices of the black unit and the color unit.
Therefore, there may be concern about the occurrence of the problem that the manufacturing cost of the members increases or the correction control method for improving the image quality in terms of the density level and the hue of an output image becomes complicated.